Can i pay zakat for my fiancee who is from a christian family and is not able de pay it by herself ?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The question is somewhat ambiguous. Do you mean that your fiancée is a Muslim but her family is Christian, or that she is still a Christian and has not yet converted to Islam?

If she is still a Christian and has not converted to Islam, you should know that Zakaatul-Fitr is obligatory only on Muslims and their Muslim dependents. It is not obligatory on a non-Muslim or his non-Muslim dependents.

If she has converted to Islam, it is permissible for you to pay Zakaah on her behalf, but only after she gives you permission to do so because Zakaah is due on her, and her intention is required. Her intention to pay the Zakaah is only held (in this case) by giving you permission to pay it on her behalf.

Allah Knows best.